A communication system using an optical fiber is an important technique for realizing long-distance and high-capacity communication. This long-distance and high-capacity communication is realized by a device for modulation/demodulation used for transmission/receiver and the wideband characteristics of an optical fiber to be a transmission path.
Recently, by utilizing the characteristics, a lot of techniques have been realized which enable an optical communication system having such a high-speed interface capacity in which the bit rate is 100 Gbps.
When performing long-distance transmission of an optical signal with a high-speed bit rate of 100 Gbps via an optical fiber transmission path, there is a problem of waveform distortion due to chromatic dispersion of the optical fiber. Chromatic dispersion is a property in which light group delay differs according to the wavelength, and a 1.5 micron band single-mode fiber has a value of 16 to 17 ps/nm/km.
Because the optical transmission pulse is broadened due to this chromatic dispersion, long-distance transmission of an optical signal is difficult. For example, in the case of a 10-Gbps NRZ (Non Return to Zero) signal, an optical signal can be transmitted only for a distance of as far as about 50 to 80 km. Transmission distances become shorter in inverse proportion to the square of bit rate due to chromatic dispersion. Therefore, in the case of a higher-speed bit rate, for example, in the case of 100 Gbps, the transmission distance of an optical signal is 1 km or less when there is chromatic dispersion.
In order to perform long-distance transmission of an optical signal with a high-speed bit rate, it is necessary to compensate for broadening of the optical transmission pulse due to this chromatic dispersion, more accurately, waveform distortion of an optical signal caused by the chromatic dispersion. In current optical communication systems, a dispersion compensation device such as a dispersion compensation fiber is used to compensate for the waveform distortion.
The dispersion compensation device is provided with a dispersion compensation amount that has an absolute value substantially equal to that of the dispersion amount of chromatic dispersion caused in a transmission path and an opposite sign. As a result, the transfer function of the dispersion compensation device is an inverse function of the transfer function of the transmission path. Hereinafter, the dispersion amount of chromatic dispersion will be referred to simply as the dispersion amount.
Since the process in which waveform distortion is caused by chromatic dispersion is a linear process, the dispersion amount caused on a transmission path is compensated for by the dispersion compensation amount given by a dispersion compensation device by connecting the transmission path and the dispersion compensation device in series. As a result, the waveform of an optical signal is restored at output of the dispersion compensation device. Thereby, even in a transmission path where chromatic dispersion occurs, long-distance transmission of an optical signal with a high-speed bit rate is realized.
In comparison, in a wavelength-division multiplexing optical network which will be widely developed in the future, route switching by an optical switch or the like is performed to realize flexible route setting. In such a wavelength-division multiplexing optical network, the dispersion amount also changes when route switching is performed.
There has been practically used a device which is called a VIPA (Virtually-Imaged Phase Array) and which enables the dispersion compensation amount to be variable in response to a change in the dispersion amount. However, the transmission distance which enables the dispersion compensation amount to be variable is as short as tens of kilometers in the case of a signal with a bit rate of 10 Gbps, and therefore, the device cannot cope with route switching accompanied by great distance fluctuation.
For example, Non Patent Literature 1 discloses a technique for compensating for the dispersion amount of chromatic dispersion by processing an electric signal in an optical-signal-transmitter-side apparatus. As described above, the process in which waveform distortion is caused by chromatic dispersion is a linear process. Therefore, the dispersion compensation device for compensating for the dispersion amount caused in a transmission path may be arranged before or after the transmission path. In the technique disclosed in Non Patent Literature 1, a dispersion compensation device is provided in the optical-signal-transmitter-side apparatus, which is positioned before a transmission path.
As an example of a dispersion compensation device, a transversal filter configured as shown in FIG. 1 can be given.
Transversal filter 10 shown in FIG. 1 is provided with multiple delay elements 11, multiple multipliers 12 and adder 13.
In the technique disclosed in Non Patent Literature 1, the dispersion amount is compensated for, for example, by the transversal filter shown in FIG. 1 for an electric signal inputted to the optical-signal-transmitter-side apparatus. An operation in which the dispersion amount is compensated for by the transversal filter shown in FIG. 1 will be described below.
Electric signal 20 inputted to the optical-signal-transmitter-side apparatus is given different delays by multiple delay elements 11 as shown in FIG. 1.
An output signal from each delay element 11 is inputted to next-stage delay element 11 and multiplier 12. The signal inputted to multiplier 12 is referred to as a branch signal.
The signal inputted to the next-stage delay element 11 is further given delay by that delay element 11. On the other hand, the branch signal inputted to multiplier 12 is multiplied by a tap coefficient outputted from circuit coefficient control device 14.
Then, the signal multiplied by the tap coefficient by each multiplier 12 is inputted to adder 13, and the sum total is determined by adder 13. As the delay interval of delay given by each delay element 11, for example, a value equal to the half of the symbol time of a signal to be transmitted is used.
The tap coefficient provided from circuit coefficient control device 14 is a value determined by an impulse response of a transfer function. Since the transfer function due to chromatic dispersion is a complex function, this tap coefficient is a complex number. Therefore, an output after compensation of the dispersion amount is also a complex signal.
Then, an optical signal is modulated with the use of complex signal 21 for which the dispersion amount has been compensated for. Actually, an IQ converter included in the transmitter-side apparatus applies the real part of complex signal 21 to the in-phase component (cosine component) of the optical signal, and the imaginary part of the complex signal to the orthogonal component (sine component) of the optical signal. An IQ converter is an apparatus for dividing an inputted signal into a signal in phase (I) and a signal with a phase orthogonal to the in-phase signal (Q).
In the technique disclosed in Non Patent Literature 1, since the transfer function can be freely changed by changing the tap coefficient outputted from circuit coefficient control device 14 to transversal filter 10, it is possible to variably compensate for the dispersion amount in a wide range.
It is also theoretically possible to apply the above technique disclosed in Non Patent Literature 1 to an optical-signal-receiver-side apparatus. In an optical receiver which is widely used at present, however, information of a complex signal is lost due to square-law detection at the time of converting an optical signal to an electric signal by a photodiode.
In comparison, in a technique disclosed in Non Patent Literature 2, each item of information regarding the in-phase component (cosine component) of the electric field of a received optical signal and information regarding the orthogonal component (sine component) is abstracted by performing coherent optical receiver to make a phase diversity receiver configuration.
By obtaining a complex electric field signal of the optical electric field of the received optical signal and processing this complex electric field signal with a transversal filter, it is made possible to compensate for the dispersion amount.
The capability of compensating for the dispersion amount is substantially the same between the case of performing the compensation on a transmitter side and the case of performing the compensation on a receiver side, within a range in which deterioration due to the nonlinear effect of the system can be ignored. However, in the case where the dispersion amount of a transmission path changes due to route switching by an optical switch or the like in a wavelength-division multiplexing optical network, waveform distortion caused by the change in the dispersion amount can be detected only on the receiver side, and the transmitter side cannot detect the waveform distortion. In the case of compensating for the dispersion amount on the receiver side, it is possible to quickly optimize the receiver state by adaptive equalization because the state of waveform distortion can be always checked on the receiver side.
Here, in compensating for the dispersion amount by a transversal filter, the number of delay elements required to compensate for the same dispersion amount and the number of branch signals outputted from the delay elements (hereinafter referred to as the number of taps) significantly increase as the bit rate of an optical signal is higher. Therefore, the scale of a circuit for compensating for the dispersion amount becomes significantly large.
As a method for preventing the scale of the circuit for compensating for the dispersion amount from becoming significantly large, there is a method in which compensating for the dispersion amount is divided into tasks to be performed on the transmitter side and on the receiver side. A technique therefore is disclosed, for example, in Non Patent Literature 3.
Non Patent Literature 3 discloses a system which compensates for the dispersion amount in an optical transmitter and an optical receiver by utilizing optical fibers. By arranging a dispersion compensation fiber for the optical transmitter and the optical receiver, the dispersion compensation amount for the optical transmitter and the optical received is decreased. This prevents the scale of the circuit for compensating for the dispersion amount from becoming large.